1. Field of the Invention
The present invention relates generally to a coding/decoding apparatus and method in an OFDM (Orthogonal Frequency Division Multiplexing) mobile communication system, and in particular, to a coding/decoding apparatus and method using an STTD (Space-Time block coding based Transmit Diversity) technique.
2. Description of the Related Art
An OFDM technique recently used for high-speed data transmission over a wire/wireless channel, a technique for transmitting data using multiple carriers, is a kind of MCM (Multi-Carrier Modulation) technique, which converts a stream of serial input symbols into parallel symbols and modulates each of the converted parallel symbols with a plurality of orthogonal subcarriers (or subchannels).
A system supporting the MCM technique, called an “MCM system,” was first applied to a high-frequency radio for military use, in the late 1950's, and research on the OFDM technique for overlapping a plurality of orthogonal subcarriers has been made from 1970's. However, due to the difficulty in realizing orthogonal modulation between multiple carriers, the OFDM technique was rarely applied to an actual system. However, after Weinstein et al. proposed in 1971 that OFDM modulation/demodulation could be efficiently performed using DFT (Discrete Fourier Transform), active research has been carried out on the OFDM technique. In addition, as a technique of using a guard interval and inserting a cyclic prefix guard interval becomes generally known, it has become possible to reduce a negative influence on the system due to multipath and delay spread interference. Therefore, the OFDM technique has been widely applied to such digital transmission techniques as DAB (Digital Audio Broadcasting), digital television, WLAN (Wireless Local Area Network), WATM (Wireless Asynchronous Transfer Mode), and fixed BWA (Broadband Wireless Access). That is, the OFDM technique was not widely used due to its hardware complicity. However, as various digital signal processing techniques including FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) have recently been developed, it has become possible to realize the OFDM technique. The OFDM technique, though similar to the conventional FDM (Frequency Division Multiplexing) technique, is characterized in that it has optimal transmission efficiency during high-speed data transmission by maintaining orthogonality between the multiple subcarriers. In addition, the OFDM technique, having high frequency utilization efficiency and strong resistance to multipath fading, is advantageous in that it has optimal transmission efficiency during high-speed data transmission. Further, the OFDM technique, since it overlaps frequency spectrums, has high frequency utilization efficiency and strong resistance to frequency selective fading and multipath fading, and can reduce inter-symbol interference (ISI) by utilizing a guard interval. In addition, it is possible to design an equalizer having a simple structure and strong resistance to impulse noises. Due to the advantages stated above, there is a growing trend for the OFDM technique to be widely used for the communication systems.
A transmitter and a receiver of a mobile communication system supporting the OFDM technique (hereinafter, referred to as “OFDM mobile communication system”) will be briefly described.
In an OFDM transmitter, input data is modulated with subcarriers through a scrambler, encoder and interleaver. The transmitter provides a variety of variable rates, and has a coding rate, an interleaving size and a modulation technique, which can be changed according to a data rate. Commonly, the encoder uses a coding rate of ½ and ¾, and an interleaving size for preventing a burst error is determined according to the number of coded bits per OFDM symbol (NCBPS). The modulation technique includes QPSK (Quadrature Phase Shift Keying), 8 PSK (8-ary Phase Shift Keying), 16 QAM (16-ary Quadrature Amplitude Modulation) and 64 QAM (64-ary Quadrature Amplitude Modulation) according to the data rate. Meanwhile, a predetermined number of pilots are added to the signal modulated with a predetermined number of subcarriers, and the pilot-added signal undergoes IFFT, generating one OFDM symbol. Thereafter, a guard interval for preventing the inter-symbol interference in the multipath channel environment is inserted in the OFDM symbol, and the guard interval-inserted OFDM symbol is finally applied to an RF (Radio Frequency) processor through a symbol wave generator, and then transmitted over a channel.
In an OFDM receiver corresponding to the transmitter, a reverse operation of the operation performed by the transmitter is performed and a synchronization process is added. First, the receiver performs a process of estimating a frequency offset and a symbol offset of a received OFDM symbol by utilizing a training symbol. Thereafter, a guard interval-eliminated data symbol is restored to a predetermined number of pilot-added subcarriers through an FFT block. In addition, in order to overcome a propagation delay phenomenon on an actual wireless channel, an equalizer estimates a channel condition of a received channel signal and eliminates signal distortion on the actual wireless channel from the received channel signal. The channel estimated data through the equalizer is converted to a bit stream, and then output as final data through a deinterleaver, a decoder for error correction, and a descrambler.
Although the OFDM technique has a strong resistance to frequency selective fading, its performance is limited. A typical example of improved general techniques proposed to overcome performance limitations is a diversity technique using multiple antennas. The diversity technique is classified into a time diversity technique, a frequency diversity technique and a space diversity technique.
The time diversity technique is generally provided by a channel coding technique combined with an interleaving technique. In the case of the time diversity technique, as a time variation of a channel becomes greater, its gain increases. Frequency diversity can be obtained by transmitting a signal with different frequencies thereby generating a multipath component of a channel. Therefore, the frequency diversity is also called “path diversity,” and a Rake receiver in a DS-CDMA (Direct Spread-Code Division Multiple Access) mobile communication system is a typical example of the frequency diversity. Space diversity can be obtained by generating independent fading channels through multiple transmission and reception antennas.
In order to improve reception performance in the OFDM mobile communication system, a number of diversity techniques have also been proposed. However, most of the diversity techniques simply combine the techniques previously proposed in other systems, rather than utilizing the unique characteristic of the OFDM mobile communication system. Accordingly, there have been demands for a method of obtaining a maximum diversity gain with low complexity by making the best use of the characteristic of the OFDM mobile communication system.
Now, a structure of a transmitter for an OFDM mobile communication system will be described with reference to FIG. 1.
FIG. 1 illustrates a structure of a transmitter in a general OFDM mobile communication system. Referring to FIG. 1, the transmitter encodes input data into coded bits at a given coding rate, and interleaves the coded bits, thus generating data 110. The generated data 110 is provided to a modulator (or QPSK/QAM mapper) 120. Although there have been proposed various coding techniques, the transmitter typically employs a coding technique using a turbo code, or an error correction code. Further, the transmitter generally uses a coding rate of ½ and ¾. The modulator 120 modulates the input data 110 by a predetermined modulation technique, and outputs modulated symbols. Here, the modulation technique includes QPSK, 8 PSK, 16 QAM and 64 QAM, and each of the modulation techniques performs modulation by its unique symbol mapping technique. It will be assumed in FIG. 1 that QPSK and QAM are used as the modulation technique. The modulated symbols output from the modulator 120 are provided to a first IFFT block 130. The IFFT block 130 generates an OFDM symbol by performing IFFT on the modulated symbol. The OFDM symbol output from the IFFT block 130 is provided to a guard interval inserter 140. The guard interval inserter 140 inserts a guard interval in the OFDM symbol output from the IFFT block 130. Transmission of the OFDM symbol is commonly performed in a block unit. However, the OFDM symbol is affected by a previous symbol, while it is transmitted over a multipath channel. In order to prevent interference between the OFDM symbols, the guard interval is inserted between consecutive blocks. The guard interval-inserted OFDM symbol from the guard interval inserter 140 is transmitted over a multipath channel through an antenna ANT after being up-converted by an RF processor 150.
Next, a structure of a receiver for an OFDM mobile communication system will be described with reference to FIG. 2.
FIG. 2 illustrates a structure of a receiver in a general OFDM mobile communication system. Referring to FIG. 2, a signal transmitted from a transmitter over a multipath channel is received at an RF processor 210 through an antenna ANT. The RF processor 210 down-converts the RF signal received through the antenna ANT into an IF (Intermediate Frequency) signal, and provides the IF signal to a guard interval eliminator 230. The guard interval eliminator 230 eliminates the guard interval inserted into the OFDM symbol output from the RF processor 210. The guard interval-eliminated OFDM symbol is provided to an FFT block 240. The FFT block 240 generates a modulated symbol through an FFT process.
If the OFDM mobile communication system uses N subcarriers, a signal output from the FFT block 240 can be represented byr(k)=H(k)X(k)+n(k), 0≦k≦N−1  Equation (1)
Equation (1) can be rewritten in a determinant, as followsr=H·X+n  Equation (2)
In Equation (2), r denotes an N×1 reception symbol vector, X denotes an N×1 transmission symbol vector, n denotes an N×1 noise vector, and H denotes an N×N diagonal matrix representing a frequency response of a channel. Since a frequency selective fading channel is expressed as a frequency ratio selective fading channel, the receiver has a good characteristic for the frequency selective fading channel having multiple paths.
As stated above, since the received symbol is represented by the simple product of a channel frequency response and a transmission signal, it is possible to restore a signal with a simple equalizer such as a one-tap equalizer 250. If it is assumed that the receiver fully recognizes the channel information, an equalization process performed by the equalizer 250 is represented by
                                          X            ^                    ⁡                      (            k            )                          =                                            r              ⁡                              (                k                )                                                    H              ⁡                              (                k                )                                              ,                                    Equation        ⁢                                  ⁢                  (          3          )                    
Equation (3) can be rewritten in a determinant, as follows{circumflex over (X)}=H−1·r  Equation (4)
A transmission symbol for the equalized signal is determined through a demodulator (or QPSK/QAM demapper) 260. The demodulator 260 demodulates the modulated symbols symbol-mapped by a predetermined modulation technique and outputs coded bits, and the coded bits are restored to an original signal through interleaving and decoding processes.
As stated above, the OFDM mobile communication system is designed to overcome the inter-symbol interference caused by the wireless channel. However, the OFDM mobile communication system is not highly resistant to signal attenuation due to a multipath phenomenon of the wireless channel. In order to prevent a performance deterioration due to the fading channel, there has been proposed an OFDM mobile communication system supporting a diversity technique. However, most of the previously proposed mobile communication systems simply combine the techniques previously proposed in other systems, rather than utilizing the unique characteristic of the OFDM mobile communication system.
That is, the OFDM mobile communication system has considerable performance degradation due to the multipath fading phenomenon. Therefore, it is necessary to use a diversity technique for overcoming such a phenomenon. However, in order to utilize the ever proposed diversity techniques in the OFDM mobile communication system, a transmitter and a receiver with a complex structure are required. In addition, since the latest space diversity technique using multiple antennas must increase the number of transmission and reception antennas, the transmitter and the receiver increase in size, and the existing system, rendering the system useless in many contexts. Accordingly, there have been demands for a diversity technique suitable for the OFDM mobile communication system by making the best use of the characteristic of the OFDM mobile communication system. In addition, there have been demands for a method of obtaining a diversity gain with reduced complexity by utilizing the unique characteristic of the OFDM mobile communication system.